Method and apparatus providing graded-index microlenses

ABSTRACT

Microlenses are fabricated with a refractive-index gradient. The refractive-index gradient is produced in a microlens material such that the refractive index is relatively higher in the material nearest the substrate, and becomes progressively lower as the layer gets thicker. After formation of the layer with the refractive-index gradient, material is etched from the layer through a resist to form microlenses. The index of refraction can be adjusted in the microlens material by controlling oxygen and nitrogen content of the microlens materials during deposition. High-oxide material has a lower index of refraction. High-oxide material also exhibits a faster etch-rate. The etching forms the material into a lens shape. After removal of the resist, the microlenses have a lower relative refractive index at their apex, where the index of refraction preferably approaches that of the ambient surroundings. Consequently, light loss by reflection at the ambient/microlens interface is reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS FIELD OF THE INVENTION

This invention generally relates to microlenses and in particular tomicrolenses having reduced reflection at an interface with an ambientenvironment.

BACKGROUND OF THE INVENTION

Microlenses are used to focus light of a larger area onto a photodiodeof a solid-state imager pixel, for example. Microlenses also can be usedto trap light into solar cells. Also, light from a light-producingcomponent can be transmitted through microlenses, for example, toproject an image for display. Advanced products and systems that utilizemicrolenses in these and other similar ways include, without limitation,digital cameras, flat-panel visual displays, and solar panels. Suchproducts and systems are used in a wide variety of practicalapplications.

The direction that light is propagated through two media, such as airand a lens, is based on the relationship between the refractive indicesof the media. Snell's Law (Eq. 1) relates the indices of refraction n ofthe two media to the directions of propagation in terms of angles to thenormal: $\begin{matrix}{\frac{n_{1}}{n_{2}} = \frac{\sin\quad\theta_{1}}{\sin\quad\theta_{2}}} & (1)\end{matrix}$The index of refraction (n) is defined as the speed of light in vacuum(c) divided by the speed of light in the medium (v), as represented byEq. 2: $\begin{matrix}{n = \frac{c}{v}} & (2)\end{matrix}$The refractive index of a vacuum is 1.000. The refractive index of airis 1.000277. Representative materials used in microlens andsemiconductor device fabrication include oxides, such as silicon dioxide(SiO₂) with a refractive index of 1.45, and nitrides, such as siliconnitride (Si₃N₄) with a refractive index of 2.0.

When light travels from a medium with a low refractive index, such asair, to a medium with a high refractive index (the incident medium),such as silicon nitride, the angle of light with respect to the normalwill increase. In addition, some light will be reflected. This willreduce the efficiency the imaging system, since not all of the lighthitting the lens will travel through the lens to the photodiode, forexample.

Light reflection that would occur at the interface between two media canbe reduced if the two media have similar indices of refraction. U.S.Pat. No. 6,833,601 to Murakami teaches semiconductor imaging devices inwhich a refractive-index matching layer is provided over photodiodes ofa solid-state imaging device. The refractive-index matching layer formedof mixed insulating-material compounds having a combined compositionrepresented as SiO_(x)N_(y). The oxygen and nitrogen contents of thelayer are varied by regulating a mixture of insulating-materials duringdeposition. The mixture is regulated to have thelowest-oxygen/highest-nitrogen combined-content in the initial deposit,adjacent the photodiode-containing layer. The refractive index of thisinitially-deposited layer is similar to that of thephotodiode-containing layer. As deposition continues, the mixture isadjusted to progressively-increase combined-oxygen content, and decreasecombined-nitrogen content. Material having thehighest-oxygen/lowest-nitrogen combined-content is deposited last, nearthe top of the refractive-index matching layer. As a result, reflectanceat an interface between the photodiode and the refractive-index matchinglayer is reduced.

A microlens with reduced reflection of incident light would capture andtransmit more light to the photosensor of a solid-state imager, forexample. The increased light captured would include light thatpreviously would have been reflected. Likewise, if the microlens wereused in a display, reduced-reflection of the display light would producea brighter display image.

BRIEF SUMMARY OF THE INVENTION

The present invention in various exemplary embodiments provides amicrolens, and associated fabricating methods, with an internalstructure having a refractive-index gradient. The refractive-indexgradient minimizes reflection of light incident at the interface of themicrolens and improves a light-receiving or light-transmittingefficiency.

The exemplary microlens can be formed on a supporting substrate bydepositing a film-stack made from combined-compound materials. Each ofthe compound materials has a different index of refraction. The mixtureof the compounds is varied under control during deposition toprogressively produce a refractive-index gradient in the film-stack. Theresulting film-stack is formed into microlenses, as described furtherbelow. Different refractive-indices are featured at different depths ofthe microlens. The refractive index of the combined-compound material atthe microlens-ambient and microlens/substrate interfaces preferably isclose to ambient or adjacent structures to which light from themicrolens is passed.

Initial film-stack deposits can use a high refractive-index material,such as silicon nitride. The refractive index of the initial film-stackdeposits closely matches the refractive index of the supportingsubstrate. The final, uppermost deposit can use a low refractive-indexmaterial, such as silicon oxide. The lowest index of refraction canapproach that of the ambient surroundings, which typically is air. Theindex of refraction for air 1. Silicon oxide has a similar refractiveindex. If additional deposits are made between the first and finaldeposits, the refractive indices of the compounds used will varyprogressively from that of silicon nitride to that of silicon oxide. Thegradient can be continuous or stepped.

As an alternative to a stacked layered structure, a single layer ofmaterial may be formed such that during the fabrication of the layer theratio of oxygen to nitrogen is changed providing a refractive indexgradient.

The deposited materials combined to make the exemplary microlenses canbe represented by SiOxNy, where x and y are non-negative numbers and areinversely related. Refractive indices of the deposited materials can beadjusted during deposition by regulating the amounts ofoxygen-containing and nitrogen-containing materials in the gaseous mixof a chemical vapor deposition apparatus. Material having a higheroxygen/lower nitrogen content exhibits a relatively-lower index ofrefraction. Material having a higher nitrogen/lower oxygen content willexhibit a relatively-higher index of refraction. The oxygen/nitrogencontents of the materials can be regulated discretely to provide alaminate-microlens construct having a stepped gradient. Theoxygen/nitrogen content also can be regulated continuously duringfabrication to provide a smooth gradient.

The exemplary microlenses can be provided in an array matrix disposedover a corresponding array of solid-state photo-active devices. Thephoto-active devices can include photocollectors such as those usedconventionally in semi-conductor imagers. The photo-active devices alsocan include light-generators, such as light-emitting diodes (LEDs), asare conventionally used in displays and projectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a first step in the fabricationof microlenses having a refractive-index gradient according to anexemplary embodiment of the present invention;

FIG. 2 illustrates a further step in the fabrication of microlenses byforming a film-stack having a refractive-index gradient according to theexemplary embodiment of the present invention;

FIG. 3 diagrams a relationship between the oxygen and nitrogen contentsof the film-stack having a refractive-index gradient illustrated in FIG.2;

FIG. 4 illustrates fabrication steps subsequent to those shown in FIGS.2 and 3, including patterning a resist and etching to form microlenseshaving a refractive-index gradient according to the exemplary embodimentof the present invention;

FIG. 5 illustrates completed microlenses having a refractive-indexgradient according to the exemplary embodiment of the present invention;

FIG. 6A illustrates initial steps in the fabrication of microlenseshaving a refractive-index gradient according to another exemplaryembodiment of the present invention;

FIG. 6B illustrates additional steps subsequent to those shown in FIG.6A in the fabrication of microlenses having a refractive-index gradient;

FIG. 7A illustrates initial steps in the fabrication of microlenseshaving a refractive-index gradient according to an additional exemplaryembodiment of the present invention;

FIG. 7B illustrates additional steps subsequent to those shown in FIG.7A in the fabrication of microlenses having a refractive-index gradient;

FIG. 8A illustrates initial steps in the fabrication of microlenseshaving a refractive-index gradient according to a further exemplaryembodiment of the present invention;

FIG. 8B illustrates additional steps subsequent to those shown in FIG.8A in the fabrication of microlenses having a refractive-index gradient;

FIG. 9A illustrates initial steps in the fabrication of microlenseshaving a continuous refractive-index gradient according to an exemplaryembodiment of the present invention;

FIG. 9B illustrates additional steps subsequent to those shown in FIG.9A in the fabrication of microlenses having a refractive-index gradient;

FIG. 10 is a block diagram of an imaging device featuring microlensesfabricated according to an exemplary embodiment of the presentinvention; and

FIG. 11 is an illustration of a computer system including a CMOS imageraccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and illustrate specificexemplary embodiments by which the invention may be practiced. It shouldbe understood that like reference-numerals represent like-elementsthroughout the drawings. These embodiments are described in sufficientdetail to enable those skilled in the art to practice the invention, andit is to be understood that other embodiments may be utilized, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The term “substrate” is to be understood as including silicon,silicon-on-insulator (SOI), or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “substrate” in the followingdescription, previous process steps may have been utilized to formregions or junctions in or over the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but could be based on silicon-germanium, germanium, or gallium arsenide,for example.

The invention can be used in conjunction with various light-managementapplications including, for example, imagers employing photosensors, anddisplay devices utilizing light-emitters. For convenience, however, andwithout limitation, the invention is described in the context of animager. A portion of a CMOS imager 10 in the initial stages ofmicrolens-fabrication is shown in FIG. 1 in cross-section. FIG. 1illustrates in simplified form two representative cells from a matrixarray of photosensing-cells that substantially completed except for acorresponding microlens matrix array.

Each photosensing cell includes a photodiode 11 separated bytrench-isolation regions 14 formed in a p-type substrate 18. Eachphotodiode 11 is constituted in part by an n-type charge-collectionregion 20 and a p-type region 22. Floating diffusion region 24 is linkedto the n-type charge-collection region by a gate 26 of a source-followertransistor. A transparent insulating layer 28 is formed over the gatesand other photosensor components to form a component layer 32. Anetch-stop layer 34 is supplied over component layer 32. The fabricationof microlenses from the film-stack 36 is described below. The locations30 where the microlenses will be formed is shown by dashed lines in FIG.1.

Those of ordinary skill in the art will understand that additionalimager components are not shown in FIG. 1. Imagers also can includeadditional transistors and gate structures, metal interconnect layers,color-filter arrays, etc. These and other details are not shown for thesake of clarity. FIGS. 2, 4, 5 include the component layer 32 showngenerically, without including any component-details, for additionalsimplicity and clarity of illustration. In similar fashion, componentlayers 50, 60, 70, 80 are shown generically in FIGS. 6A, 6B; 7A; 7B; 8A,8B; 9A, 9B respectively. It is understood that the component layers 32,50, 60, 70, 80 include solid-state and other imager components, and thatthe invention is not limited to photosensors, nor to a particularphotosensor arrangement. Additional details on specific layers of animager between the photosensors and fabricated membranes are shown inrepresentative U.S. Pat. Nos. 6,844,580, 6,784,013, which areincorporated herein by reference.

Microlens-fabrication according to an exemplary embodiment of thepresent invention proceeds with formation of a graded-index film-stack36. Film-stack 36 is illustrated generically in FIG. 1. Referring toFIG. 2, the graded-index film-stack 36 includes several layers of amixture of insulating material deposited by low-pressure chemical vapordeposition (CVD). According to an exemplary embodiment of the presentinvention plasma-enhanced CVD (PECVD) is utilized.

Fabrication of the graded-index film-stack 36 is performed using amixture of inorganic microlens materials. At least one of the microlensmaterials-is a compound that includes oxygen, such as SiO2. Anothermicrolens materials is a compound that includes nitrogen, such as Si3N4.The amounts of the oxygen and nitrogen compounds being mixed areregulated during deposition to produce the desired gradient. Theelemental content of the mixed-compound graded-index film-stack can berepresented as SiOxNy.

By adjusting the oxygen and nitrogen content of the deposited materialduring deposition, various properties of the deposited material in thefilm-stack can be controlled. For example, as the amount of oxygen inthe microlens material increases, the refractive index of the materialdecreases. In addition, as the oxygen content increases, the etch-rateof the material increases. Concomitantly, as the nitrogen contentincreases, the refractive index increases, and the etch rate decreases.The refractive-index and etch-rate profile of the microlens iscontrolled by regulating the amount of SiO2 and Si3N4 in the depositedmicrolens material.

According to an exemplary embodiment, material first-deposited on thesubstrate 42, represented as SiOxNy, can have an oxygen content as lowas x=0. The amounts of oxide and nitride are controlled to increase theoxygen content of the deposited material. Material in the last-depositedportions 40 of the graded-index layer 36 can have a nitrogen content aslow as 0. Between layers 40 and 42 are several layers havingprogressively higher oxygen content as deposition proceeds from thefirst-deposited material to the last-deposited material. In theillustration of FIG. 2, the low-oxide (high-nitride) material is shownwith darker shading; the high-oxide (low-nitride) material is shown withlighter shading.

The deposition process is exemplified by plasma enhanced chemical vapordeposition (PECVD) techniques in which gaseous reactors are used to formsolid layers on a substrate surface. Deposition is enhanced by the useof a vapor containing electrically charged particles or plasma, at lowertemperatures. The compositions and amounts of the materials beingdeposited can be adjusted in the gaseous reactors to regulate the typesand properties of the materials being deposited.

FIG. 3 diagrams an example of the relationship between the oxygen andnitrogen contents of the film-stack 36 represented by SiOxNy. In FIG.3A, the oxygen (O2) and nitrogen (N2) contents (%) are shown on theordinate, and the deposition position in the deposition direction,refractive index, and etch-rate of the film-stack 36 are shown on theabscissa. Raw material gases such as SiH4, NH3, O2, and the like can beused.

The film-stack 36 is deposited by using PECVD or other low-pressure CVDapparatus. During deposition an oxygen gas flow rate can be controlledas an increasing function. At the same time, a nitrogen (NH3) gas flowrate can controlled as a decreasing function. In this way, in thefilm-stack represented by SiOxNy, x increases from the initial depositto the last deposit, and y decreases respectively.

Therefore, in the film-stack 36, the refractive index can be varied fromthe refractive index of a silicon oxide film (1.45) to the refractiveindex of a silicon nitride film (2.0), as viewed in the direction oflight incident on the film-stack. The etch-rate also varies. Both therefractive index and the etch-rate can vary continuously or step-wise.In a continuous-variation construct, described in connection with FIGS.9A, 9B below, multiple reflection is decreased to improve lightreceiving sensitivity, as compared with the step-wise case in which thefilm-stack is essentially only two layers—a single layer of siliconnitride and a single layer of silicon oxide, as discussed below inconnection with FIGS. 6A, 6B.

After the graded film-stack 36 is deposited, a resist layer 38 is formedover the graded-index film-stack 36 and patterned. Etching takes placethrough patterned openings in the resist layer 38. High-oxide microlensmaterial exhibits a higher etch rate than the low-oxide material, asrepresented by the horizontal arrows in FIG. 3. Consequently, as etchingproceeds microlenses 44 having a refractive-index gradient are formed asshown in FIG. 4. The resist layer 38 is removed to obtain themicrolenses 44 as shown in FIG. 5.

The following chart illustrates etch chemistry and oxide andnitride-etch rates for two film-stack materials representative ofetch-rates and the graded film-stacks of an exemplary embodiment of thepresent invention: AA:HF AA:HF APM/SC1 APM/SC1 Film-Stack 100:1 30:1100:3:2_55 20:4:1_65 BOE 20:1 HF HF 10:1 TEOS PECVD 19.1 49.3 2540 >37000 632 Si3N4 7.9 23.4 1 59 1655 159

In the chart, the various etchants can be identified as follows: AA:HF100:1 represents 100 parts Acetic Acid:1 part Hydrofluoric Acid (by wt%). AA:HF 30:1 represents 30 parts Acetic Acid:1 part Hydrofluoric Acid(by wt %). APM/SC1 100:3:2_(—)55 represents APM=AmmoniumHydroxide/Hydrogen Peroxide Mixture; SC1=Standard Clean 1 (same as APM);100:3:2_(—)55=100 parts DIW:3 parts H2O2:2 parts NH4OH at 55° C. (by wt%). APM/SC1 20:4:1_(—)65 represents APM=Ammonium Hydroxide/HydrogenPeroxide Mixture; SC1=Standard Clean 1 (same as APM); 20:4:1_(—)55=20parts DIW:4 parts H2O2:1 parts NH4OH at 65° C. (by wt %). BOE 20:1represents BOE (Buffered Oxide Etchant) ˜49 wt % of 20 parts ammoniumfluoride:1 part HF, the make up is DIW (by wt %). HF is pure HF. HF10:1—Hydrofluoric Acid—10 parts DIW:1 part HF (by Wt %)

Referring to FIGS. 6A, 6B, 7A, 7B, 8A, 8B the present invention isexemplified by various alternative methods of microlens fabrication inwhich film-stacks having discrete layers represented as SiOxNy areutilized. FIGS. 6A, 6B feature an imager component layer 50 supportingtwo CVD layers 52, 54, each layer being. Uppermost glass layer 54 is ahigh-oxide, high etch-rate layer as compared to layer 52, which has alower oxide concentration. FIG. 6B illustrates schematically theresulting microlens structure after patterning and wet-etching thelayered film-stack of FIG. 6B and removal of the patterned resist. Afteretching, less remains of glass layer 50, which etches more rapidly thanlayer 52. Layer 52 has been etched away less than layer 50. The etchedlayers 50, 52 approximate the shape of a lens. The index of refractionfor layer 50 is closer to that of ambient. Consequently, light receivedthrough layer 50 will transmit more-readily into the microlens structurethan had it encountered only layer 52, which is made of material havinga higher index of refraction than layer 50. The etch-rate and index ofrefraction are adjusted in the two layers by regulating the amount ofoxide and nitride deposited in each layer, as described above. Aphotoactive structure, such as a photodiode, and associated circuitrydeveloped in substrate 50 combine with the microlens structure to forman imager pixel 59.

Referring next to FIGS. 7A and 7B, a microlens fabrication is shown inwhich a substrate 60 supports a film-stack featuring three layers 62,64, 66. Layers 62, 64 may be of similar composition as layers 52, 54 ofFIGS. 6A and 6B, for example. An intermediate layer 66 has etch-rate andindex-of-refraction values between those provided by layers 62, 64. Inthe etched microlens structure illustrated in FIG. 7B, light passingfrom lens layer 64 to lens layer 62 passes through intermediate layer66. The refractive index of layer 66 is intermediate those of layers 62,64, and so lowers the relative difference in the refractive indices ateach interface between layer 62, 66, and layers 66, 64. Consequently,reflective light-loss is reduced as compared to the microlens structureof pixel 59 shown in FIG. 6B. A photoactive structure, such as aphotodiode, and associated circuitry developed in substrate 50 combinewith the microlens structure to form an imager pixel 69.

A further-refined microlens fabrication is illustrated in FIGS. 8A and8B. A substrate 70 supports a film-stack featuring four depositedmicrolens layers 72, 74, 76, 78, as shown in FIG. 8A. The film-stack hasbeen patterned and etched to form a microlens structure of a pixel 79 asshown in FIG. 8B. The two intermediate layers 76, 78 have etch-rates andindices of refraction between those of slow-etch layer 72 and high-etchlayer 74. The etch-rate of layer 76 is slower than that of layer 78. Themicrolens structure of pixel 79 features a more-gradual change in therefractive indices from layer to layer than was provided in microlensstructures of pixels 59, 69. The microlens structure of pixel 79 alsomore-closely approximates a lens in profile.

FIGS. 9A and 9B illustrate a microlens fabrication for acontinuously-graded microlens of a pixel 89. FIG. 9A shows substrate 80supporting continuously-graded microlens film-stack 82. The film-stack82 is deposited using CVD techniques in which the material compositionis set to provide a slow-etch, high refractive-index deposition at theoutset on substrate 80. As deposition continues, the materialcomposition is regulated to have increasingly-faster etch-rates andhigher refractive indices. Rather than having distinct layers, thefilm-stack 82 features a continuous gradation of properties. Patterningand etching takes place as described above. The resulting microlens 89has a smooth lenticular profile. The exemplary microlens of the pixel 89can be formed as one of an array or matrix of microlenses, for example,associated with an array of photo-imaging devices.

FIG. 10 illustrates a CMOS imager incorporating an imaging device array500 having pixel cells using a lens structure in accordance with theinvention to focus light onto the pixel cells. The pixel cells of eachrow in the imaging device array 600 are all turned on at the same timeby a row select line, and the pixel cells of each column are selectivelyoutput by respective column select lines. A plurality of row and columnlines is provided for the entire array 600. The row lines areselectively activated in sequence by the row driver 610 in response torow address decoder 620. The column select lines are selectivelyactivated in sequence for each row activation by the column driver 660in response to column address decoder 670. Thus, a row and columnaddress is provided for each pixel cell. The CMOS imager is operated bythe control circuit 650, which controls address row address decoder 620and column address decoder 670 for selecting the appropriate row andcolumn lines for pixel readout, and row driver 610 and column driver660, which apply driving voltage to the drive transistors of theselected row and column lines.

The pixel output signals typically include a pixel reset signal Vrsttaken off of a floating diffusion region (via the source followertransistor) when it is reset and a pixel image signal Vsig, which istaken off the floating diffusion region (via the source followertransistor) after charges generated by an image are transferred to it.The Vrst and Vsig signals are read by a sample and hold circuit 661 andare subtracted by a differential amplifier 662, which produces adifference signal (Vrst−Vsig) for each pixel cell, which represents theamount of incident light. This difference signal is digitized by ananalog to digital converter 675. The digitized pixel signals are thenfed to an image processor 680 to form and output a digital image.

FIG. 11 shows a system 900, a typical processor system modified toinclude an imager device (such as the CMOS imager device 500 illustratedin FIG. 10) of the invention. The processor system 900 is exemplary of asystem having digital circuits that could include image sensor devices.Without being limiting of the invention, such a system could include acomputer system, camera system, scanner, machine vision, vehiclenavigation, video phone, surveillance system, auto focus system, startracker system, motion detection system, image stabilization system, andother systems employing an imager, other light-collecting applications,and light-projection or display applications.

System 900, for example a camera system, generally comprises a centralprocessing unit (CPU) 902, such as a microprocessor, that communicateswith an input/output (I/O) device 906 over a bus 904. CMOS imager device500 also communicates with the CPU 902 over the bus 904. Theprocessor-based system 900 also includes random access memory (RAM) 910,and can include removable memory 914, such as flash memory, which alsocommunicate with the CPU 902 over the bus 904. The CMOS imager device608 may be combined with a processor, such as a CPU, digital signalprocessor, or microprocessor, with or without memory storage on a singleintegrated circuit or on a different chip than the processor.

Exemplary embodiments of the invention have been described with specificreferences to pixels, photodiodes, and imaging devices. The inventionhas broader applicability and may be used in any imaging apparatus. Forexample, the invention may be used in conjunction with any solid-stateimager, for example, charge coupled device (CCD) imagers, solar panels,and display devices. In general, the invention has applicability toimage-formation devices using microlenses. More generally, the inventionprovides structure and fabrication methods in which a microlens has agraded index of refraction. Although convex microlenses have beenillustrated and described, those of skill in the art will appreciatethat other microlens configurations, such as concave microlenses, alsocould be fabricated by applying the disclosed teachings.

The processes and devices described above illustrate exemplary methodsand devices out of many that could be used and produced according to thepresent invention. The above description and drawings illustrateexemplary embodiments which achieve the objects, features, andadvantages of the present invention. It is not intended, however, thatthe present invention be strictly limited to the above-described andillustrated embodiments. Any modification, even if presentlyunforeseeable, of the present invention that comes within the spirit andscope of the following claims should be considered part of the presentinvention.

1-7. (canceled)
 8. A solid-state imaging device comprising a photoelectric transducer and an associated lens, the lens having an apex and a base, the lens comprising a lens-material having a changing index of refraction.
 9. A solid-state imaging device as in claim 8, wherein an oxygen content of the lens-material varies inversely with a nitrogen content of the lens material with respect to a thickness of the lens from the base to the apex of the lens.
 10. A solid-state imaging device as in claim 9, wherein the oxygen content is highest toward an apex of the lens.
 11. A solid-state imaging device as in claim 9, wherein the nitrogen content is highest toward a base of the lens.
 12. A solid-state imaging device as in claim 9, wherein the oxygen content x is zero toward the base of the lens.
 13. A solid-state imaging device as in claim 9, wherein the nitrogen content y is zero toward the apex of the lens.
 14. A solid-state imaging device as in claim 8, wherein the lens material is arranged in discrete layers.
 15. A solid-state imaging device as in claim 8, wherein the lens material is arranged in continuous layers.
 16. A solid-state imaging device as in claim 8, wherein the lens is convex.
 17. A lens structure comprising: a substrate containing a photodevice; and a microlens having a base and a thickness measured from the base, formed of oxygen and nitrogen-containing materials disposed over the substrate and in optical communication with the photodevice, a ratio of oxygen-content to nitride-content in the materials disposed on the substrate increasing with the thickness of the microlens.
 18. A lens structure as in claim 17, wherein the microlens material is inorganic.
 19. A lens structure comprising: a substrate supporting a photodevice; and a microlens having a base and having a thickness measured from the base, and formed of a microlens material disposed over the substrate and in optical alignment with the photodevice, an index of refraction of the microlens material decreasing with the thickness of the microlens.
 20. A microlens array comprising a plurality of microlenses, each microlens comprising a microlens material disposed on the substrate to provide a thickness measured from the substrate, an index of refraction of the microlens material decreasing with the thickness of the microlens.
 21. A microlens array as in claim 20, wherein the array if provided in an imaging device.
 22. A microlens array as in claim 21, wherein the imaging device is part of a processing system.
 23. A method of forming a microlens comprising steps of: forming a lens-material including oxygen and nitrogen and having an etch-rate gradient; coating and patterning a resist on the film-stack; selectively etching the film-stack material through openings in the resist; and removing the resist.
 24. A method as in claim 23, wherein the etch-rate gradient is obtained by adjusting the oxygen content and the nitrogen content of the lens-material during deposition of the lens-material.
 25. A method as in claim 23, wherein the etch-rate gradient is formed such that lens-material has a progressively higher etch-rate nearer the resist.
 26. A method as in claim 25, wherein the oxygen:nitrogen content ratio is relatively higher toward the top of the lens material than toward the bottom of the lens material.
 27. A method of fabricating a microlens comprising steps of: layering material on a substrate to form a film-stack having a bottom and a top defining a thickness, the film-stack having a gradient; patterning a resist layer formed on the film-stack; and removing the material from the film-stack selectively through openings in the resist layer such that relatively more of the material is removed at respectively thicker portions of the film-stack, such that remaining material forms a microlens.
 28. A method as in claim 27, wherein the step of removing comprises etching using a wet etch.
 29. A method as in claim 27, wherein the material has an oxygen:nitrogen content ratio that varies during deposition of the material.
 30. A method as in claim 29, wherein an oxygen:nitrogen content ratio of the deposited material is varied continuously during deposition.
 31. A method as in claim 29 wherein the oxygen:nitrogen content ratio of the deposited material is varied in discrete steps during deposition. 